1. Field of the Invention
The present invention relates generally to a method and apparatus for forming an image with a computer illustration system, and more particularly, to a method and apparatus for minimizing the scope of operation of layered synthetic graphics filters in an image.
2. Description of Related Art
Generally, computerized two-dimensional illustration systems provide designers with tools for constructing a design by layering graphical objects while specifying their properties. Examples of commercial computerized two-dimensional illustration systems are CorelDraw.TM. by the Corel Corporation, FreeHand.TM. by the MacroMedia Corporation, and Adobe Illustrator.RTM. by the Adobe Corporation. In such illustration systems, the properties of a graphical object may vary depending on the particular graphical object being developed. For example, bounded geometrical shapes are graphical objects with properties that include line width, fill color, and transparency whereas line segments are graphical objects with properties that include line width and line color.
A characteristic of illustration systems is that they tie the properties of each graphical object closely to the manner in which each object is constructed. This close tie makes the creation of complex visual effects using these illustration systems difficult. The following examples illustrate the close tie between a graphical object's construction and its properties. First, filled regions must be explicitly constructed and closed. Second, line and fill properties must be applied to complete outlines or complete line segments. Finally, transformations can only be applied to complete objects or groups of objects. This property of illustration systems is cumbersome for designers to create visual effects that appear to cross object boundaries. In order to create a visual effect that appears to cross object boundaries, a designer typically must introduce extra shapes and boundaries into a design. As a result, designers that operate these systems are limited because they must create object boundaries that define the extent of any visual effect.
Spatially bounded synthetic graphics (hereinafter "SG") filters or lenses enable a designer to define a graphical object with line or vector graphical properties that change depending on the particular objects in the scene in which they are defined. SG filters (or lenses) have been developed in order to minimize the effect of the close tie between the properties of a graphical object and the manner in which the object is constructed. Unlike non-lens graphical objects which have an appearance that does not change once it is defined (i.e., static), bounded lenses change the appearance of objects seen through their interiors. These bounded lenses therefore have an appearance in a scene that is "dynamic." Unlike static graphical objects, dynamic objects have properties that change as objects are added to or taken from the bounded region of the lens. In effect, these spatially bounded lenses enable visual effects to extend across object boundaries which are independent of the boundaries of each of the underlying objects.
Examples of spatially bounded lenses (or SG filters) are disclosed in U.S. Pat. Nos. 5,596,690, 5,467,441, and 5,479,603, which are assigned to the same assignee as the present invention and incorporated herein by reference. Particular reference is made to U.S. Pat. No. 5,479,603 which describes a method for composing overlapping lenses in an original image. The method described in U.S. Pat. No. 5,479,603 composes, or combines, the functions of multiple viewing operations operating on a model data structure to produce a single composite image displayed in the context of an original image. Also, commercial illustration systems such as Adobe Illustrator, MacroMedia FreeHand, and CorelDraw support a rich set of tools for constructing and filling outlines. Specifically, the CorelDraw software contains a number of lenses (or SG filters) which are described in a section entitled "Lens" in CorelDRAW.TM. User's Manual--Vol. 1--Version 5.0, Corel Corporation, 1994, pp. 274-297.
Related to spatially bounded filters are planar maps, and tools that operate strictly on raster images. Planar maps, which are disclosed by Gangnet et al. in "Incremental Computation Of Planar Maps," Proceedings of SIGGRAPH '89 (Boston, Mass., August), Computer Graphics, Vol. 23, No. 3, pages 345-354, have been used to provide a way to partition existing scene geometry into regions that can be filled. Some of the effects disclosed by Gangnet et al. may appear similar to lenses. The difference between lenses and planar maps is that planar maps make the object partitioning easy. Lenses, however, make object partitioning unnecessary. Furthermore, lenses can also create effects beyond those provided by planar maps by adding and deleting objects, and performing affine transformations. Also, tools that operate strictly on rasters may create lens-like effects on raster objects, however, these raster tools are unable to operate on geometric objects.
The aforementioned systems, however, do not compose objects in a scene that are defined as lenses (or SG filters) in varying orders that depend on a lens type identified for each lens. With each lens having a lens type, an ordered list of objects in a scene can be either composed in a front-to-back or back-to-front order. Such a capability would enable the creation of a wider variety of scenes with respect to each lens. Accordingly, it would be desirable to provide a generic system for efficiently composing lenses (or SG filters), defined as either front-to-back or back-to-front lenses, that overlap with other objects in a scene.